efficacy of Norgestomet implants on performance and preventing pregnancy in grazing postpubertal beef heifers

A 240-d study was conducted to assess the efficacy of long-acting, erodible implants containing Norgestomet (NOR; 11β-methyl-17α-acetoxy-19-norprogesterone) on growth performance and preventing pregnancy in grazing postpubertal beef heifers. Nonpregnant, cyclic crossbred beef heifers (n = 240) were stratified by BW (763 ± 7.5 lb [346 ± 3.40 kg]) and assigned to 80 blocks (3 heifers/block). Blocks were allocated randomly to 5 pastures (48 heifers/pasture). Heifers were assigned randomly within block to receive 1 of 3 implants (randomized complete block design) containing 0 (control), 100, or 150 mg NOR (d 0). Heifers grazed mixed fescue-bermudagrass pastures with ad libitum access to mineral-vitamin mix. Heifers were exposed to fertile bulls from d 7 to 200 (≥ 1 bull/25 heifers). Bull breeding soundness evaluations were conducted on d 0, 50, 100, and 150. Heifer BW was recorded pretreatment and on d 50, 100, 150, 200, and 240. Pregnancy was diagnosed by ultrasonography on d 50, 100, 150, 200, and 240. Orthogonal contrasts were used to assess the effects of control vs NOR (100 or 150 mg NOR) and dose efficacy (100 vs 150 mg NOR). Control heifers exhibited lower (P < 0.05) average daily gain (ADG) when compared to heifers that received 100 or 150 mg NOR (0.84 vs 1.04 and 1.06 lb [0.38 vs 0.47 and 0.48 kg]/d, respectively). Implants did not completely eliminate pregnancy at any NOR dosage. Pregnancy rates were 90.0 (90% CI = [82.9, 94.4]), 8.9 (90% CI = [4.8, 15.8]), and 12.5% (90% CI = [7.5, 20.0]) for control, 100, or 150 mg NOR, respectively. Heifers that received 100 or 150 mg NOR experienced lower (P < 0.0001) pregnancy rates compared to control heifers. By d 150, 88.8% (71/80) of control heifers were pregnant, and 90.0% (72/80) were pregnant by d 200. Administration of 100 or 150 mg NOR prevented pregnancy in at least 92.5% of heifers for 200 d. Pregnancy rates of heifers that received 100 or 150 mg NOR were not different (P = 0.46). These data suggest that implants containing 100 or 150 mg NOR increased rate of gain and decreased pregnancy rates in grazing postpubertal beef heifers.